During the past dozen years, plastic shopping bags, and particularly those of what are called the T-shirt type, have largely replaced paper bags in grocery stores and supermarkets. The number of T-shirt bags which are sold annually in the United States alone approaches 100 billion.
Many varieties of T-shirt bags have been devised by various inventors and, almost from the outset, efforts have been made to provide bag packs which, when mounted on a conventional twin arm rack, have the back wall of each bag of the pack detachably adhered to the front wall of the immediate following bag. The purpose of this detachable adhesion is to cause the back wall of the preceding bag, after it is filled and it is being drawn off the rack by its mounting handles, in effect, to pull the front wall of the next succeeding bag forward from its back wall and thereby open the mouth of the bag while the bag arms are still hung on the rack so the thus opened bag may be filled with purchased items by the store sales clerk or box boy.
Initially, the desired adhesion has been suggested to be accomplished by providing some adhesive spot or limited adhesive area between the back wall of each lead bag and the front wall of the next ensuing bag. Examples of such detachable adhesive bondings may be found in U.S. Pat. Nos. 4,676,378; 5,020,750; 5,307,935; 5,333,730; 5,469,970; and 5,561,967.
A second method for producing the desired detachable adhesion between the rear wall of one bag and the front wall of the next ensuing bag has been by applying pressure forces on selected spots or limited areas of the bag pack when the pack of blanks is subjected to the cutting die. An example of such method of providing the desired detachable adhesion may be seen in U.S. Pat. No. 5.465,846, and is suggested in U.S. Pat. No. 5,183,158. The pressure point expedient, however, may also be utilized in conjunction with either adhesive, or with the third method of providing adhesion, namely, corona treatment. Examples of the latter combination may be found in U.S. Pat. Nos. 5,335,788; 5,562,580; 5,507,713; and 5,125,604.
As explained in U.S. Pat. No. 5,335,788, while corona treatment may provide suitable adhesion between bags made of low density polyethylene (LDPE) or linear low density polyethylene (LLDPE), as disclosed in U.S. Pat. No. 5,087,234, sufficient adhesion has not been found to occur where high molecular weight high density polyethylene (HDPE) is utilized in the manufacture of the bag. It is for this reason that the inventors of U.S. Pat. No. 5,335,788 have additionally provided in that patent for pressure points or at least one localized compressed area in combination with corona treatment of HDPE bags. These last mentioned inventors have stated in their patent that there was insufficient adhesion for HDPE bags where the bags, during formation, had been subjected only to a single corona exposure and the pressure of the die as it came down on packs of blanks to cut out the bag mouths. It has been found necessary, therefore, in prior corona treated bags to supplement whatever adhesion may occur during bag cutting, with special pressure points accomplished by incorporating some type of staking element or elements in the blank cutting die.
Effecting detachable adhesion by applying adhesive to the bag walls has always presented problems in bag manufacture as, for example, the adhesive must be of a particular consistency and state and applied accurately to limited areas and, after it has been applied, the bag pack must be maintained at temperatures and other conditions which will not cause the adhesive to harden or to become too soft. Either of these results could present problems in securing the desired opening of the bags of the bag pack. Other disadvantages in use of adhesive bags are aptly described in column 2 of U.S. Pat. No. 5,465,846.
As to relying upon instruments to effect pressure adhesion at a selected point or points in the bags of the bag pack, this requires modification of the normal cutting die and, unless carefully placed and properly calculated for the quantum of pressure applied, can result in so weakening of the bags at such pressure points that the bags may tear at or about such pressure points when filled with groceries or purchased articles at the store and separation of the adhered walls is effected by pulling the leading bag from the rack arms.
It may be seen, therefore, that a need still exists for providing bag packs having a consistent type of detachable adhesion between the rear and front walls of consecutive bags with pressure points of minimum impact on the bag packs to avoid weakening walls of the bags in the pack with the resulting tearing of bags about such pressure points.